Cardiac arrhythmias, such as atrial fibrillation, are commonly occurring disorders characterized by erratic beating of the heart. The regular pumping function of the atria is replaced by a disorganized, ineffective quivering caused by chaotic conduction of electrical signals through the upper chambers of the heart. Atrial fibrillation may be caused by the rapid and repetitive firing of an isolated center or focus within the atrial cardiac muscle tissue. Such foci may act to trigger AF or may, once triggered, sustain the fibrillation. Recent studies have suggested that foci for such arrhythmia are predominantly in a region of cardiac tissue proximal to the pulmonary veins that extend out of the heart from the left atrium. More particularly, tissue proximal to the superior pulmonary veins denoted as ostia are likely AF foci.
While medication can be an effective treatment for some cases, many patients are not responsive to medical therapies and require alternative treatment. As an alternative to medication, a surgical technique, known as the Maze technique, requires open chest surgery to strategically incise the atrial wall, and subsequently repair the incisions by suturing. The result of this surgery is to create scar tissue located along the incision lines and extending through the atrial wall to block electrical conductivity from one segment to another.
While the Maze procedure has proven effective in restoring normal sinus rhythm, it requires considerable prolongation of cardiopulmonary bypass and aortic crossclamp time, especially when performed in combination with other open heart procedures. Over the last decade, more simplified techniques have been proposed which replace surgical incisions with ablations, or scars, formed in the heart tissue. The various energy sources used in ablation technologies include cryogenic, radiofrequency (RF), laser, and microwave energy. The ablation devices are used to create tissue lesions in an affected portion of the heart in order to block electrical conduction.
One common ablation technique employs the use of a catheter that is introduced into the heart (e.g., intravascularly) to direct RF energy at specific areas of heart tissue found to be the source of the irregular rhythms. An electrophysiology (EP) study is first performed to discover the location and characteristics of the arrhythmia and, once the specific location is identified and mapped, RF energy is delivered to the tissue to ablate the tissue, thus forming a lesion that blocks electrical conduction. While minimally invasive techniques are usually preferred, the procedure is often performed in combination with other open heart procedures as a prophylactic to prevent post-operative onset of atrial fibrillation.
RF ablation techniques are typically successful in treating atrial fibrillation, however the lesions must be well defined within the heart to be effective. The lesion must have a sufficient length, continuity and/or depth to interrupt or to block electrical conduction across the affected portion of the heart. This can be difficult to achieve without forming an incision in the atrium. In addition, if the energy is not uniformly transmitted to the target site, hot spots can form, possibly leading to severe tissue damage or blood coagulation (clots).
One potential problem that may be encountered during cardiac ablation procedures is the risk of collateral tissue damage. In some cases the energy-delivering electrode performing the ablation is positioned at the purported focus and a pad, which acts the return electrode, is externally placed on the patient's body. Although most of the generated energy may be appropriately directed at the focus, the uncertain, unpredictable energy return path from the heart to the return electrode pad may lead to damage of other vital organs or structures. The esophagus, the lungs, and nerve tissue are examples of organs or tissue structures that may be susceptible to unintended energy influx.
Accordingly, there exists a need for ablation systems and methods that can be used safely and effectively to effect cardiac ablation procedures.